The present invention is directed to moisture separator-reheaters for steam power generation.
Moisture separator-reheaters for steam power generation typically employ large cylindrical shells containing moisture separators and heat transfer tubes extending therethrough. The separators are of the inertial type and separate water from wet steam exhausted from the high pressure turbine. The steam is then directed to the heat exchange portion of the unit. The heat transfer tubes of the heat exchange unit are arranged in one or more bundles and employ throttle steam to reheat the main steam flow. The throttle steam is ideally controlled such that all of the tube-side steam is condensed but not subcooled. The separated shell-side water is then exhausted from the bottom of the unit while dry steam is passed to the low pressure turbine. Such drying and reheating of the low pressure steam improves overall system efficiency and reduces moisture to the low pressure turbine. Only a limited number of inlets can advantageously be used for introducing the moist steam into such units. Because of the size of these units, the volume of flow of the influent and the pressure drop between the inlets and outlets, it is difficult to obtain uniform flow through both the inertial separators and the heat exchange tube bundles. European practice has been to vertically dispose such moisture separator-reheaters and introduce steam at one end. Horizontally disposed moisture separator-reheaters in the United States have also been designed with an inlet at one end.
Because of the length and horizontal disposition of American type moisture separator-reheaters, it is believed that a significant portion of the flow passes directly from the inlet to the inertial separators and onto the tube bundles without first becoming distributed along the total length of the vessel. This improper distribution results in both inefficient separation of moisture and inefficient heating of the dried steam.
The substantial length of such separators, the variations in temperature throughout the unit at steady state conditions and the rapid heating and cooling which may occur during nonsteady state conditions have created major problems in the design and operation of horizontally disposed moisture separator-reheaters. Reheater tube bundles with vertically disposed U-bends can experience damaging thermal stress during nonsteady state conditions because of large variations in temperature caused by the entry of hot or cold flow into either the inlets or outlets of the vessel.
A common problem with horizontally disposed moisture separator-reheaters is the unequal heat transfer occurring in the tubes of each tube bundle. The lowermost tubes are subjected to a high temperature differential while tubes higher in the tube bundle receive shellside flow which has already been partially heated by the lower tubes. As a result, the lower tubes tend to accumulate water until they no longer carry steam along their entire length. Subcooling of the water in the lower tubes may then occur while steam passes through the entire length of tubes higher up in the bundle. Such occurrences create an unstable two-phase condition resulting in cyclical filling and emptying of the lower tubes with water. This condition leads to less efficient overall heat transfer and cyclical thermal stresses on the tubes and tube sheet.
Another problem experienced by moisture separator-reheaters is the flashing of shell-side water to steam as it is passing from the system. Such flashing can damage components, throw water into the reheater bundles and eventually through the steam outlets, create surges of steam to the low pressure turbine and generally detract from the efficient operation of the moisture separator-reheater.